1. Field of the Invention
The present invention relates to the process of photolithography employed in the manufacturing of semiconductor devices. More particularly, the present invention relates to a phase shift mask and to a method of fabricating the same.
2. Description of the Related Art
A technique known as photolithography is used in the manufacturing of semiconductor devices. A conventional photolithographic process begins with the forming of a photoresist layer on a semiconductor substrate. Then, the photoresist layer is selectively exposed to light that has been passed through a photo mask bearing a mask pattern, whereby a pattern corresponding to the mask pattern is transferred to the photoresist. The exposed photo resist is developed, whereby a photoresist pattern, also corresponding to the pattern of the photo mask, is formed. Then, exposed regions of a layer underlying the photoresist pattern can be etched using the photoresist pattern as a mask, whereby various elements such as interconnections or electrodes are formed.
As semiconductor devices become more highly integrated, conventional photolithographic processes must be enhanced or changed to form finer and finer patterns. Various methods have been suggested to this end. For example, one method is to use an electrical beam, an ion beam or X-rays to expose the photoresist. Another known method is an off-axis illumination method that relies on diffracting the exposure light. Other methods concentrate on refinements to the composition of the photoresist. Still further, another known method capable of forming fine patterns is an exposure method using a phase shift mask.
A phase shift mask has a phase shifter that is configured to diffract light passed through the phase shift mask, whereby the resolution or depth of focus of the exposure process is increased. More specifically, the substrate and the phase shifter of the mask act to produce a phase difference in light transmitted by the mask. The difference between the light transmitted through the substrate only and the light transmitted through the phase shifter can be expressed as θ=2πt (n−1)/λ (wherein n is the index of refraction of the phase shifter, t is the thickness of the phase shifter, and λ is the wavelength of the exposure light). A phase shift mask wherein θ is equal to π is known as a reverse phase shift mask because the light transmitted through respective portions of the mask has a phase difference of 180°. In this case, the diffraction of the light by the phase shift mask gives rise to interference that causes the intensity of the light to become zero at a boundary of the pattern transferred to the photoresist layer. As a result, the pattern has a high degree of contrast. Adopting the exposure method using a phase shift mask to improve the resolution of a conventional photolithographic process only involves the fabricating of the phase shift mask. That is, a phase shift mask can be readily incorporated into existing photolithographic equipment. Thus, this method is relatively easy to adopt in comparison with the other methods mentioned above.
FIG. 1 is a cross-sectional view of a conventional phase shift mask. The conventional phase shift mask includes an opaque pattern 12 disposed on the surface of a mask substrate 10. The opaque pattern 12 is formed from a layer of chromium. A first transparent region 20 and a second transparent region 18 are formed between sections of the opaque pattern 12. The second transparent region 18 is constituted by a recess/recesses in the mask substrate 10 formed by etching the substrate. The recesses, in turn, constitute the phase-shifter of the mask. That is, light transmitted through the second transparent region 18 is 180° out of phase with respect to light transmitted through the first transparent region 20. Therefore, the phase shift mask of FIG. 1 can be considered as possessing a non-phase shifting region (I), a phase shifting region (II) and an opaque region (III).
FIG. 2 is a graph illustrating a limitation in using a conventional phase shift mask. The pitch of patterns formed by exposing photoresist patterns using a conventional phase shift mask is plotted along the abscissa of the graph, whereas the size of the patterns is plotted along the ordinate. In this example, the desired size, i.e., dimension, of the pattern is 80 nm.
Referring to FIG. 2, the conventional phase shift mask can be used to produce a pattern having the desired size; however, for this to occur the pitch of the pattern produced exceeds a certain value. That is, patterns having the desired size of approximately 80 nm can be produced only if the pitch of the pattern is at least 500 nm. If it is desired to produce a pattern having a pitch of 400 nm or less, the size of the pattern must be dramatically greater or less than 80 nm.
Accordingly, the opaque pattern 12 of the phase shift mask must be reduced if the mask is to be capable of being used to form patterns having the same size but at relatively small pitches. For instance, in this example, the critical dimension of the opaque pattern should be less than 50 nm to be able to produce patterns from 40 nm to 80 nm at a pitch of 400 nm or less. However, it is difficult to form a phase shift mask in which the opaque pattern has a CD of less than 50 nm.